The impact of two extensively-modified rivers (Georges and Cooks Rivers) on a receiving basin exhibiting minor ecological risk (Botany Bay) (Australia)

Abstract

Botany Bay is a large, tide-dominated, drowned river valley on the eastern seaboard of Australia, which was initially planned as the first European settlement in the country. The Georges River has a large catchment (936.5 km 2) and comprises mainly undeveloped parkland (62%), while the Cooks River catchment is small (103 km 2) and substantially modified (90 %). The extent of anthropogenic change and level of ecologic risk were determined for these areas using sedimentary metals as an initial screening assessment of environmental condition of these waterbodies. Cooks River was assessed as one of the most anthropogenically-modified urban rivers on the east coast of Australia with sediments exhibiting severe metals enrichment and moderate ecologic risk. Georges River sediments were highly enriched with moderate to slight ecological risk. Both Georges and Cooks Rivers discharge to Botany Bay, which was mantled in highly metal-enriched sediments, but exhibited only minor ecological risk. These results illustrated an unusual example of an estuary significantly influenced by metal-enriched sediments, but exhibiting minimal ecological risk. This anomaly was due to high anthropogenic-metal concentrations in the fine fraction of surficial sediment, which were considerably diluted by abundant metal-poor coarse material resulting in low total metal concentrations and reduced adverse ecological risk. The low proportion of fine material in surficial sediment was possibly due to increased ambient wave and tidal energy in the bay. Size-normalised metal distributions show distinct dispersion pathways from Cooks and Georges Rivers to the mouth of Botany Bay. Stormwater was a major source of metals to the system, supplemented by leakage from waste dumps and secondary sourcing by remobilisation of enriched-legacy sediments.